Problems in molecular orbital theory and valence bond theory
Answers
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Because arguments based on atomic orbitals focus on the bonds formed between valence electrons on an atom, they are often said to involve a valence-bond theory.
The valence-bond model can't adequately explain the fact that some molecules contains two equivalent bonds with a bond order between that of a single bond and a double bond. The best it can do is suggest that these molecules are mixtures, or hybrids, of the two Lewis structures that can be written for these molecules.
This problem, and many others, can be overcome by using a more sophisticated model of bonding based on molecular orbitals. Molecular orbital theory is more powerful than valence-bond theory because the orbitals reflect the geometry of the molecule to which they are applied. But this power carries a significant cost in terms of the ease with which the model can be visualized.
Molecular orbital theory :
1. It regards the nuclei of the molecules as polycentric and constructs the molecular orbitals.
2.The Atomic orbitals lose their individual identity while combining.
3. It takes into account only the increase or the decrease in the electron density at over lapping.
4.The number of bonds is half the difference in the bonding and anti-bonding electrons.
5. It helps in predicting the paramagnetic or diamagnetic character of the molecules.
6. In this theory resonance is not at all considered.
Valency Bond theory :
1. Electrons present in the molecule are localised as if they are in isolated atoms.
2.The Atomic orbitals of the combining atoms retain their individual identity.
3. It involves only the increase in the electron density in the region of orbital overlap.
4.The number of covalent bonds formed is equal to the number of combining orbitals.
5.This theory doesn't help in predicting the paramagnetic or diamagnetic behaviour of molecules.
6. It is easy to apply.
7. In this theory resonance is considered.